Previous studies in this laboratory have focused on the molecular characterization of two mammalian sarcoma viruses, the Harvey and Kirsten sarcoma viruses. These viruses are known to contain genetic information found to be endogenous to the rat in addition to a portion of the parent mouse leukemia virus. This project was designed to investigate the expression of the rat endogenous viral genetic information during the development of chemically induced rat mammary tumors in order to determine if endogenous virus expression may be involved in the chemical carcinogenesis process.